MAX8686ETL+T ,Single/Multiphase, Step-Down, DC-DC Converter Delivers Up to 25A Per PhaseApplications REFINMAX8686RS-POL Power SuppliesPHASE/REFO CS+Module ReplacementsCOMP CS-POKTelecom E ..
MAX8686EVKIT+ ,Evaluation Kit for the MAX8686FeaturesThe MAX8686 evaluation kit (EV kit) provides a prov- S Operates from a 6V to 20V Input Supp ..
MAX8688AHETG , Digital Power-Supply Controller/Monitor with PMBus Interface
MAX8688ALETG , Digital Power-Supply Controller/Monitor with PMBus Interface
MAX868EUB ,Regulated / Adjustable -2x Inverting Charge PumpELECTRICAL CHARACTERISTICS(V = +3.3V, SHDN = IN, C1 = C2 = 0.22µF, C = 1µF, C = 10µF, T = 0°C to +8 ..
MAX868EUB ,Regulated / Adjustable -2x Inverting Charge PumpApplications MAX868C/D 0°C to +70°C Dice*MAX868EUB -40°C to +85°C 10 µMAXSmall LCD Panels*Dice are ..
MB84VD21183EM-70PBS , Stacked MCP (Multi-Chip Package) FLASH MEMORY & SRAM CMOS
MB84VD21194EM-70PBS , Stacked MCP (Multi-Chip Package) FLASH MEMORY & SRAM CMOS
MB84VD22181FM-70PBS , 32M (X16) FLASH MEMORY & 4M (X16) STATIC RAM
MB84VD22182EE-90 ,32M (x 8/x16) FLASH MEMORY & 4M (x 8/x16) STATIC RAMFUJITSU SEMICONDUCTORDS05-50204-2EDATA SHEETStacked MCP (Multi-Chip Package) FLASH MEMORY & SRAMCMO ..
MB84VD22183EE-90 ,32M (x 8/x16) FLASH MEMORY & 4M (x 8/x16) STATIC RAMFEATURES• Power supply voltage of 2.7 to 3.3 V• High performance90 ns maximum access time (Flash)85 ..
MB84VD22184FM-70 , 32M (X16) FLASH MEMORY & 4M (X16) STATIC RAM
MAX8686ETL+T
Single/Multiphase, Step-Down, DC-DC Converter Delivers Up to 25A Per Phase
General DescriptionThe MAX8686 current-mode, synchronous PWM step-
down regulator with integrated MOSFETs operates from
a 4.5V to 20V input supply and generates an adjustable
output voltage from 0.7V to 5.5V while delivering up to
25A per phase.
The MAX8686 employs a peak current-mode architec-
ture that operates with an adjustable switching frequency
from 300kHz to 1MHz. An adjustable current-limit thresh-
old allows for optimization for different applications with
different load currents. Inductor current sense is
achieved either using an external sense resistor or using
a lossless inductor current-sense scheme. The foldback
and hiccup current limit reduces the power dissipation
during overload or short-circuit conditions and allows for
autorecovery when the fault condition is removed.
The MAX8686 offers the ability to start up monotonically
even when there is a prebias output voltage. In addi-
tion, an adjustable soft-start capability allows for a con-
trolled turn-on. The MAX8686 features an accurate 1%
reference and offers a reference input that allows for a
higher accuracy reference to be used for voltage track-
ing applications such as DDR memory.
The MAX8686 can be paralleled (up to eight) together
in a true multiphase mode to deliver up to 200A of out-
put current. When operating in this mode, this device
achieves better than 10% current balance between
phases at full load. The MAX8686 supports program-
mable phase shedding to improve system efficiency
during light load conditions.
Other features include an enable input and a power-OK
(POK) indicator used for power sequencing. The
MAX8686 also features latch overvoltage protection
that turns on the low-side MOSFET when the output
voltage exceeds 120% of the nominal voltage. The
MAX8686 is offered in a thermally enhanced 40-pin,
6mm x 6mm TQFN package.
ApplicationsPOL Power Supplies
Module Replacements
Telecom Equipment
Networking Equipment
Servers
DDR Memory
FeaturesOperating Range from 4.5V to 20V Input Supply1% Reference Voltage Accuracy Over TemperatureReference Input (REFIN) for Output Tracking or
System Reference VoltageAdjustable Switching Frequency from 300kHz
to 1MHzSingle/Multiphase Operation Delivers Up to
25A/200A with Integrated MOSFETsAdjustable Current LimitMonotonic Output Voltage at Startup (Prebias)Output Sink and Source Current CapabilityAdjustable Soft-StartThermal-Overload ProtectionOutput Overvoltage ProtectionThermally Enhanced 6mm x 6mm TQFN
Package (4W)
MAX8686
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per PhasePOK
OUTPUT
ENABLE
INPUT
BST
VIN = 12V
VOUT = 1.2V/25A
PGND
PHASE/REFO
COMP
EN/SLOPE
FREQSSGNDILIM
MAX8686REFINRS+
RS-
CS+
CS-
POK
ypical Application Circuit
Ordering Information19-4113; Rev 1; 10/10
PARTTEMP RANGEPIN-PACKAGEMAX8686ETL+-40°C to +85°C40 TQFN-EP*
+Denotes a lead-free package.
*EP = Exposed pad.
Pin Configuration appears at end of data sheet.
MAX8686
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per Phase
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VIN = VINA= 12V, VL = AVL, VREFIN = 1V, VRS+- VRS-= 1V, VRS-= 0V, VEN/SLOPE= 1.25V, VCS+= VCS-= 1V, RILIM= 122kΩ, CVL= 1μF,
CAVL = 0.22μF, CFREQ= 270pF, TA= +25°C, unless otherwise noted.) (Note 2)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
IN, INA to PGND.....................................................-0.3V to +22V
BST, DH to LX...........................................................-0.3V to +6V
BST to PGND..........................................................-0.3V to +28V
LX to PGND...........................-0.3V to (VIN+ 0.3V) (-2V for 50ns)
BST to VL................................................................-0.3V to +22V
AVL to GND.................................................-0.3V to (VVL+ 0.3V)
COMP, ILIM, FREQ, PHASE/REFO, RS+, RS-, POK, REFIN,
CS+, CS- to GND..................................-0.3V to (VAVL+ 0.3V)
VL to PGND..............................................................-0.3V to +6V
EN/SLOPE to GND...................................................-0.3V to +6V
RTN to PGND to GND to GFREQ..........................-0.3V to +0.3V
IN Continuous Current.....................................................20ARMS
LX Continuous Current....................................................25ARMS
Continuous Power Dissipation (TA= +70°C) (Note 1)
40-Pin TQFN (derate 50mW/°C above +70°C)..........4000mW
θJC(thermal resistance from junction to exposed pad)
(Note 1) ......................................................................3.5°C/W
θJT(thermal resistance from junction to top) (Note 1) ...3.9°C/W
Operating Temperature Range...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
Soldering Temperature (reflow).......................................+260°C
PARAMETERCONDITIONSMINTYPMAXUNITS
GENERALOperating Input-Voltage RangeVINA = VIN , TA = -40°C to +85°C620V
Operating Input-Voltage RangeVIN = VINA = VVL = VAVL, TA = -40°C to +85°C4.55.5V
TA = +25°C450IN/INA Shutdown Supply CurrentVEN/SLOPE = 0V,
VIN = VINA = 20VTA = +85°C500μA
IN/INA Quiescent Supply CurrentVRS+ = 1.1V, no switching; VIN = VINA = 20V,
TA = -40°C to +85°C5.56.6mA
Rising, TA = -40°C to +85°C4.24.354.45AVL Undervoltage Lockout Trip
LevelFalling4.03V
VL Output Voltage6V ≤ VIN = VINA ≤ 20V, 1mA ≤ IVL ≤ 30mA,
TA = -40°C to +85°C5.25.45.5V
SOFT-START (SS)SS Shutdown ResistanceVEN/SLOPE = 0V (master mode)20100Ω
SS Soft-Start CurrentVSS = 0.4V and 1.1V, TA = -40°C to +85°C192531μA
PHASE COMPARATOR AND REFERENCE (PHASE/REFO)Reference Output VoltageMeasured at PHASE/REFO (master mode),
TA = -40°C to +85°C3.2673.3003.333V
PHASE Comparator OffsetVRS- = VAVL (slave mode), VPHASE = 0.3V and 2.5V,
TA = -40°C to +85°C -20+20mV
REFIN INPUTREFIN Input Bias CurrentVREFIN = 0.7V or 3.3V-500+500nA
REFIN Input Voltage Range03.3V
Note 1:Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four-layer
board. For detailed information on package thermal considerations see /thermal-tutorial.
MAX8686
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per Phase
ELECTRICAL CHARACTERISTICS (continued)(VIN = VINA= 12V, VL = AVL, VREFIN = 1V, VRS+- VRS-= 1V, VRS-= 0V, VEN/SLOPE= 1.25V, VCS+= VCS-= 1V, RILIM= 122kΩ, CVL= 1μF,
CAVL = 0.22μF, CFREQ= 270pF, TA= +25°C, unless otherwise noted.) (Note 2)
PARAMETERCONDITIONSMINTYPMAXUNITS
ERROR AMPLIFIERVREFIN = 3.3V3.2673.33.333Remote-Sense Accuracy
(Including Error Amplifier Offset)
Measure as VRS+ - VRS-
(TA = -40°C to +85°C)VREFIN = 0.7V0.6930.70.707V
TransconductanceTA = -40°C to +85°C1.11.72.6mS
COMP Source CurrentVRS+ - VRS- = 1.3V220300μA
COMP Sink CurrentVRS+ - VRS- = 0.7V220300μA
COMP Shutdown ResistanceVEN/SLOPE = 0V20100Ω
RS+/RS- Input Leakage Current0.21.5μA
RS+ Input Common-Mode RangeVIN = VINA = VVL = VAVL = 4.5V, VRS- = 100mV03.4V
RS- Input Common-Mode Range-100+100mV
CURRENT-SENSE AMPLIFIERInput Offset VoltageMeasure at CS+ and CS-, VCS+ = VCS- = 0.7V and 5.5V
(TA = -40°C to +85°C)-1.5+1.5mV
Current-Sense Amplifier GainVCS- = 0 to 5V, VCS+ - VCS- = 30mV,
TA = -40°C to +85°C29.030.532.0V/V
Input Bias CurrentVCS+ = VCS- = 5.5V and 0V-4+4μA
CURRENT LIMITILIM Output CurrentVILIM = 2V, TA = -40°C to +85°C 91011μA
RILIM = 122kΩ162023Current-Limit ThresholdMeasure as VCS+ - VCS-
(TA = -40°C to +85°C)RILIM = 275kΩ384552mV
COMP Clamp Voltage HighRILIM = 275kΩ, VREFIN = 3.3V, VRS+ - VRS- = 2V3.63.84.0V
COMP Clamp Voltage LowVREFIN = 3.3V, VRS+ - VRS- = 3.35V0.540.60.66V
Maximum Peak Positive Current
Threshold RILIM = 275kΩ, no slope compensation54mV
MAX8686
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per Phase
ELECTRICAL CHARACTERISTICS (continued)(VIN = VINA= 12V, VL = AVL, VREFIN = 1V, VRS+- VRS-= 1V, VRS-= 0V, VEN/SLOPE= 1.25V, VCS+= VCS-= 1V, RILIM= 122kΩ, CVL= 1μF,
CAVL = 0.22μF, CFREQ= 270pF, TA= +25°C, unless otherwise noted.) (Note 2)
Note 2:Specifications to TA= -40°C are guaranteed by design and not production tested.
PARAMETERCONDITIONSMINTYPMAXUNITS
OSCILLATOR (FREQ)Source CurrentVFREQ = 2V, TA = -40°C to +85°C480500520μA
CFREQ = 180pF0.811.2MHzSwitching FrequencyCFREQ = 580pF240300360kHz
Minimum On-Time100ns
FREQ Discharge Resistance1050Ω
Ramp Peak Voltage2.60VAVL/22.85V
SLOPE COMPENSATION (EN/SLOPE)VSLOPE Range1.252.50V
SLOPE Source Current81012μA
THERMAL PROTECTIONThermal ShutdownRising temperature160°C
Thermal-Shutdown Hysteresis30°C
POWER-OK (POK)VOUT rising879093POK ThresholdVOUT falling87%VOUT
POK Output Voltage LowVRS+ - VRS- = 0.8V, IPOK = 2mA25200mV
POK Leakage CurrentVPOK = 5.5V0.0011μA
OVERVOLTAGE OUTPUT PROTECTION (OVP)Overvoltage Fault Trip LevelVREFIN = 3.3V, VRS+ rising, percentage of VOUT in
regulation115120125%
ENABLE (EN/SLOPE)EN Logic-High1.2V
EN Logic-Low 0.7V
BSTInternal PMOS On-Resistance8Ω
MAX8686
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per Phase
SINGLE-PHASE EFFICIENCY
vs. LOAD CURRENTMAX8686 toc01
LOAD CURRENT (A)
EFFICIENCY (%)15510
VOUT = 3.3V
VOUT = 1.8V
VOUT = 2.5V
AIRFLOW = 300 LFM
SINGLE-PHASE EFFICIENCY
vs. LOAD CURRENT (VIN = 5V)MAX8686 toc02
LOAD CURRENT (A)
EFFICIENCY (%)15510
VOUT = 1.2V
VOUT = 1.8V
VOUT = 2.5V
CIRCUIT OF FIGURE 3
AIRFLOW = 300 LFM
EFFICIENCY vs. LOAD CURRENTMAX8686 toc03
LOAD CURRENT (A)
EFFICIENCY (%)14811
270kHz
379kHz
467kHz
564kHz
AIRFLOW = 300 LFM
SWITCHING FREQUENCY
vs. CFREQ CAPACITANCEMAX8686 toc04
CFREQ CAPACITANCE (pF)
SWITCHING FREQUENCY (kHz)
CLOSED-LOOP FREQUENCY RESPONSE
(IOUT = 160A, 8 PHASES)
FREQUENCY (kHz)
GAIN (dB)100
PHASE (DEGREES)
11k
MAX8686 toc05
PHASE
GAIN
OUTPUT VOLTAGE vs. OUTPUT CURRENTMAX8686 toc06
OUTPUT CURRENT (A)
OUTPUT VOLTAGE (V)15510
AIRFLOW = 300 LFM
OUTPUT VOLTAGE vs. INPUT VOLTAGEMAX8686 toc07
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)181716151413121110987
IOUT = 10A
OUTPUT LOAD TRANSIENTMAX8686 toc08
100μs/div
VOUT
200mV/div
AC-COUPLED
IOUT
42A/div
CIRCUIT OF FIGURE 4
AIRFLOW = 300 LFM
SOFT-START WITH EN CONTROL
(IOUT = 10A)MAX8686 toc09
1ms/div
2V/div
2V/div
POK
5V/div
VOUT
500mV/div
CIRCUIT OF FIGURE 4
Typical Operating Characteristics(VIN= 12V, fSW= 500kHz, single phase, circuit of Figure 2, unless otherwise noted.)
MAX8686
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per Phase
Typical Operating Characteristics (continued)(VIN= 12V, fSW= 500kHz, single phase, circuit of Figure 2, unless otherwise noted.)
SOFT-START WITH EN CONTROL
(IOUT = 10A)MAX8686 toc10
1ms/div
2V/div
2V/div
POK
5V/div
VOUT
500mV/div
CIRCUIT OF FIGURE 4
SHUTDOWN WITH EN CONTROL
(IOUT = 10A)MAX8686 toc11
200μs/div
2V/div
5V/div
POK
5V/div
VOUT
500mV/div
CIRCUIT OF FIGURE 4
SHUTDOWN WITH EN CONTROL
(IOUT = 100A)MAX8686 toc12
200μs/div
2V/div
5V/div
POK
5V/div
VOUT
500mV/div
CIRCUIT OF FIGURE 4
SHORT-CIRCUIT PROTECTIONMAX8686 toc13a
400μs/div
IOUT
42A/div
500mV/div
POK
5V/div
VOUT
500mV/div
SHORT-CIRCUIT RECOVERYMAX8686 toc13b
400μs/div
IOUT
42A/div
500mV/div
POK
5V/div
VOUT
500mV/div
CURRENT-SHARING ACCURACYMAX8686 toc14
PHASE CURRENT (A)
CIRCUIT OF FIGURE 4
AIRFLOW = 300 LFM
LX_ SWITCHING WAVEFORM FOR
PHASES 1, 2, 3, AND 4MAX8686 toc15
200ns/div
LX1
5V/div
LX2
5V/div
LX3
5V/div
LX4
5V/div
CIRCUIT OF FIGURE 4
PHASE SHEDDING FROM 6 PHASES
TO 2 PHASES (IOUT = 30A)MAX8686 toc18
1μs/div
2V/div
LX1
10V/div
LX5
10V/div
LX2
10V/div
CIRCUIT OF FIGURE 4
MAX8686
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per Phase
PHASE RECOVERY FROM 2 PHASES
TO 6 PHASES (IOUT = 30A)MAX8686 toc19
1μs/div
2V/div
LX1
10V/div
LX5
10V/div
LX2
10V/div
CIRCUIT OF FIGURE 4
SAFE OPERATING AREAMAX8686 toc20
AMBIENT TEMPERATURE (°C)
IOUT
(A)7565703540455055603085
400 LFM
300 LFM
200 LFM
100 LFM
0 LFM
TJ = +125°C, VOUT = 1.2V
SAFE OPERATING AREAMAX8686 toc21
AMBIENT TEMPERATURE (°C)
IOUT
(A)7565703540455055603085
400 LFM
300 LFM
200 LFM
100 LFM
0 LFM
TJ = +125°C, VOUT = 3.3V
LX_ SWITCHING WAVEFORM FOR
PHASES 4, 5, 6, AND 1MAX8686 toc16
200ns/div
LX4
5V/div
LX5
5V/div
LX6
5V/div
LX1
5V/div
CIRCUIT OF FIGURE 4
REFO OUTPUT vs. TEMPERATUREMAX8686 toc17
TEMPERATURE (°C)
REFO OUTPUT (V)706050403020100-10-20-30
Typical Operating Characteristics (continued)
(VIN= 12V, fSW= 500kHz, single phase, circuit of Figure 2, unless otherwise noted.)
MAX8686
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per Phase
Pin Description
PINNAMEFUNCTIONCS-Negative Differential Current-Sense Input. Connect CS- to the output side of the inductor for lossless
current sense or to the load side of the current-sense resistor.CS+Positive Differential Current-Sense Input. Connect CS+ to the inductor through an RC network for
lossless current sense or to the inductor side of the current-sense resistor.GFREQCFREQ Capacitor Return Terminal. Connect the frequency-setting capacitor CFREQ to GFREQ as close
as possible to the device.EN/SLOPE
Enable and Slope Compensation Input. Connect a resistor from EN/SLOPE to GND to set the desired
slope compensation ramp voltage. An internal 10μA current source pulls up EN/SLOPE. The device
shuts down when the voltage at EN/SLOPE is less than 0.7V. Connect EN/SLOPE to an open-drain or
open-collector output for system enable or phase-shedding function.
5, 16LXInductor Connection. LX is high impedance during shutdown.RTNPower Ground for Low-Side Gate Driver. Connect RTN to PGND plane at the return terminal of the IN
bypass capacitor.
7–15PGNDPower Ground. Low-side MOSFET source connection.N.C.No Connection. Not internally connected.
18–26INPower Input. Connect IN to the input voltage source. Connect input bypass capacitor from IN to PGND
as close as possible to the device. Connect IN, INA, and VL together for 5V operation (see Figure 3).INAInput of the Internal VL Linear Regulator. Bypass INA with a 0.1μF capacitor to PGND.GNDAnalog GroundAVLInput Voltage to the Device’s Internal Analog Circuitry. Connect AVL to VL through a lowpass RC filter.VL
Internal 5.4V Linear Regulator Output. Connect a ceramic capacitor of at least 1μF from VL to RTN. INA
is the input to this linear regulator. Connect VL to INA when VINA is less than 5.5V. VL provides power
for the MOSFET drivers.BSTBoost Capacitor Connection. Connect a 0.22μF ceramic capacitor from BST to LX.POK
Power-Good Output. POK is an open-drain output that is high impedance when the output voltage is at
its nominal regulated voltage. The POK rising threshold is 90% of the reference voltage at REFIN. POK
is internally pulled low during shutdown. Connect POK to GND for slave mode operation.
MAX8686
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per Phase
Pin Description (continued)
PINNAMEFUNCTIONFREQ
Frequency-Setting Input. Connect a capacitor from FREQ to GFREQ to set the switching frequency.
The triangle ramp runs between 0 and AVL/2. In multiphase applications, connect FREQ of the master
and all slave devices together. FREQ is internally pulled to GFREQ during shutdown.SS
Soft-Start Input. For master-mode or single-phase operation, connect a capacitor from SS to GND to
set the soft-start time. A 25μA internal current source charges the capacitor. SS is pulled to GND in
shutdown. Connect SS to GND for slave mode operation.ILIM
Analog Programmable Current Limit. Connect a resistor from ILIM to GND to set the current limit. A
10μA current source through this resistor sets the current-limit threshold. In multiphase applications,
connect ILIM of the master and all slave devices together.REFIN
Voltage Error-Amplifier Reference Input. For master-mode or single-phase operation, connect REFIN to
PHASE/REFO through a resistor-divider to set the output voltage from 0 to 3.3V. To use an external
reference, connect REFIN to the system reference voltage, and use an RC network at REFIN to
implement soft-start if the external reference does not provide this function. Connect REFIN to GND for
slave mode operation.P H AS E /RE FO
Phase Selection Input/Reference Voltage Output. For single-phase or master-mode operation, the 3.3V
output with 1% accuracy can be used as a reference voltage. For multiphase operation, connect
PHASE/REFO of each slave device to the center tap of a resistor-divider from the master AVL to GND.
The resistor values are selected to set phase delay between phases. The PWM cycle starts 60ns after
the rising edge of VFREQ crosses VPHASE.COMP
Compensation and Output of the Voltage-Error Amplifier. Connect a Type II compensation network at
COMP. COMP is internally pulled to GND in shutdown. In multiphase applications, connect COMP of
the master and all slave devices together.RS+
Positive Input of the Output-Voltage Remote Sense. For master-mode or single-phase operation,
connect RS+ to the output-voltage sense point at the load. Connect RS+ to AVL (slave) for slave mode
operation.RS-Negative Input of the Output-Voltage Remote Sense. For master-mode or single-phase operation,
connect RS- to the remote ground at the load. Connect RS- to AVL (slave) for slave mode operation.GND_EPGround Exposed Paddle. Connect GND_EP to GND.IN_EPInput Exposed Paddle. Connect IN_EP to IN.LX_EPLX Exposed Paddle. Connect LX_EP to LX.
MAX8686
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per Phase500μA
TO S4
PGND
RTN
10μA
LEVEL
SHIFT
BSTINA
UVLO
THERMAL
SHDN
RAMP
GENERATOR
5.4V LDO
GENERATOR
PWM
CONTROL
LOGIC
CURRENT-LIMIT
CONTROL LOGIC
VOLTAGE
REFERENCE
SLAVE MODE
DETECTION
SLAVE MODE = S1, S2, S3, S5, S6 OPEN
MASTER MODE = S1, S2, S5, S6 CLOSE
10μA
AVL
FREQ
PHASE/REFO
VREF
SOFT-START
CIRCUITRY
EN/SLOPE
EN/SLOPE
AVL
CLOCK
GENERATOR
REFINFB
POK
EN/SLOPE
ERROR
AMPLIFIERPWM
COMPARATOR
PEAK CURRENT-
LIMIT COMPARATOR
AVL
AVL
VSUM
REFIN
RS+
RS-
COMP
CS+
CS-
SLOPE
COMP
1/2
CURRENT-
SENSE
AMPLIFIER
ILIM
GND
MAX8686
GFREQ
Functional Diagram
Detailed Description
DC-DC Converter Control ArchitectureThe MAX8686 step-down regulator uses a PWM, cur-
rent-mode control scheme. An internal transconduc-
tance amplifier establishes an integrated error voltage.
The heart of the PWM controller is a PWM comparator
that compares the integrated voltage-feedback signal
against the amplified current-sense signal plus an
adjustable slope-compensation ramp, which is
summed with the current signal to ensure stability. At
each rising edge of the internal clock, the internal high-
side MOSFET turns on until the PWM comparator trips
or the maximum duty cycle is reached. During this on-
time, current ramps up through the inductor, storing
energy in the inductor while sourcing current to the
output. The current-mode feedback system regulates the
peak inductor current as a function of the output-voltage
error signal. The circuit acts as a switch-mode transcon-
ductance amplifier and pushes an output LC filter pole
normally found in a voltage-mode PWM to a higher fre-
quency. See the Functional Diagram.
During the second half of the cycle, the internal high-side
MOSFET turns off and the internal low-side MOSFET
turns on. The inductor releases the stored energy as the
current ramps down, providing current to the load. The
output capacitor stores charge when the inductor cur-
rent exceeds the required load current and discharges
when the inductor current is lower, smoothing the volt-
age across the load. Under soft-overload conditions,
when the peak inductor current exceeds the selected
current limit (see the Current-Limit Circuitsection), the
high-side MOSFET is turned off immediately and the
low-side MOSFET is turned on and remains on to let the
inductor current ramp down until the next clock cycle.
Under severe-overload or short-circuit conditions, the
foldback/hiccup current limit is enabled to reduce
power dissipation.
The MAX8686 operates in a forced-PWM mode. The
converter maintains a constant switching frequency,
regardless of load, to allow for easier filtering of the
switching noise.
Internal Linear Regulator (VL)The MAX8686 contains an internal LDO regulator that
provides a 5.4V supply for the MOSFET gate drivers.
Connect at least a 1μF ceramic capacitor from VL to
RTN. VL also provides power to the internal analog cir-
cuit through AVL. Connect an RC lowpass filter (R =
10Ω, C = 0.22μF) from VL to AVL.
Undervoltage LockoutWhen AVL drops below 4.03V, the MAX8686 assumes
that the supply voltage is too low to make valid deci-
sions, so the undervoltage-lockout (UVLO) circuitry
inhibits switching and turns off both power MOSFETs.
When AVL rises above 4.35V, the regulator enters the
startup sequence and then resumes normal operation.
When operating in a multiphase configuration, the AVL
of all the devices must exceed the UVLO threshold
before any switching begins. This is achieved through
the shared ILIM pin, which is pulled low in UVLO.
Startup, Soft-Start, and Prebias OperationThe internal soft-start circuitry gradually ramps up the
reference voltage in order to control the rate of rise of
the output voltage and reduce input surge currents dur-
ing startup. The soft-start time is determined by the
value of the capacitor from SS to GND and is approxi-
mately equal to 50ms per microfarad of the capacitor.
In addition, the MAX8686 features monotonic output-
voltage rise (prebias); therefore, both power MOSFETs
are kept off if the voltage between the remote sense
input (RS+, RS-) is higher than the voltage at REFIN.
This allows the MAX8686 to start up into a prebiased
output without pulling the output voltage down.
Before the MAX8686 can begin the soft-start and
power-up sequence, the following conditions must be
met: AVLexceeds the 4.35V UVLO threshold, EN is at
logic-high, and the thermal limit is not exceeded.
Reference Output
(PHASE/REFO)/Reference Input (REFIN)The reference voltage REFO can be used to set the out-
put voltage by scaling this voltage down with a resistive
divider and using it as the input voltage to the reference
input, REFIN. The 3.3V reference voltage is 1% accurate
over temperature and can source up to 20μA.
The reference input REFIN allows the reference value of
the device to be set by an external reference. In most
applications, the 3.3V voltage with 1% accuracy from
the PHASE/REFO pin should be used as the reference.
This can be achieved by dividing the 3.3V voltage to
the desired output voltage.
MAX8686
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per Phase
MAX8686For using an external reference on REFIN, SS needs to
be tied to REFIN either directly or indirectly through a
resistor for soft-start. For REFIN voltage lower than
1.25V, connect a resistor between SS and REFIN such
that the voltage drop across the resistor due to the soft-
start current (31μA max) coming out of SS, causing the
final SS voltage to be at least 1.25V (see Figure 1a).
The external reference should be able to sink at least
31μA. Calculate RREFINas follows:
where VEXT is the external reference voltage.
In a multiphase converter, only REFIN of the master
device is connected to a reference voltage, and the
REFIN of all slave devices should be tied to GND.
The REFIN also allows for coincident voltage tracking of
multiple converters during power-up/power-down by
applying the same voltage on REFIN of the master
device in each converter.
Enable, Phase Shedding, and
Slope Compensation Input (EN/SLOPE)An internal 10μA current source pulls the EN/SLOPE
input high. The device shuts down when the voltage at
the EN/SLOPE falls below 0.7V. By connecting an
open-drain or open-collector switch to the EN/SLOPE,
this pin can be used to enable/disable a single-phase
or multiphase converter system.
A separate system signal can be used to shed some
phases of the converter at light load to eliminate all the
power loss from these phases and thus improve the sys-
tem efficiency. The phase shedding signal is connected
to the EN/SLOPE pins of the slave devices to be shed.
The right timing of the phase shedding signal from the
system is critical for the safe operation of the multiphase
converter. Only after the load current drops below a cer-
tain level, should the phase shedding signal become
high. When the open-drain or open-collector switch is
logic-low, it shuts down the slave phases connected to
the switch to reduce power loss. Before the load current
increases to a certain level, the phase shedding signal
should become logic-high to release the EN/SLOPE of
these slave devices, thus turning these phases back on
again to prepare for the higher load current. A minimum
load of 2A per phase in the remaining phases is
required for the shedded phase(s) to turn on.
The transfer function of the power stage is different with a
different number of phases. As the number of phases
increases, the power stage gain increases. The compen-
sation network should be designed such that the convert-
er is always stable with the maximum number of phases.
The EN/SLOPE input is also used to set the slope com-
pensation ramp voltage by connecting a resistor from this
input to GND. The slope compensation is used to stabi-
lize the converters when the duty cycle is more than 40%.
High-Side Gate-Drive Supply (BST)A flying capacitor between BST and LX generates the
gate-drive voltage for the internal high-side n-channel
MOSFET. When the low-side MOSFET is turned on, the
capacitor is charged by VL to 5.4V minus the drop
across the internal boost switch. When the low-side
MOSFET is turned off, the stored voltage of the capaci-
tor is stacked above LX to provide the necessary turn-
on voltage (VGS) for the high-side MOSFET. An internal
switch between BST and the internal high-side MOSFET’s
gate closes to turn the MOSFET on.
Current-Sense AmplifierThe current-sense circuit amplifies the differential current-
sense voltage (VCS+- VCS-). This amplified current-sense
signal and the internal-slope-compensation signal are
summed (VSUM) together and fed into the PWM com-
parator’s inverting input. The high-side MOSFET is turned
on by the clock in the device and is shut off when VSUM
exceeds the error-amplifier output voltage (VCOMP) at
the noninverting input of the PWM comparator. The dif-
ferential current sense is also used to provide peak
inductor current limiting. The limit can be set by adjust-
ing the analog current-limit input (ILIM).
The current-sense amplifier is used to measure the cur-
rent across the inductor by connecting to the inductor
through an RC network for lossless current sensing or
connecting to a current-sense resistor for higher accu-
racy. The input common-mode voltage range of the
current-sense amplifier is from 0 to 5.5V.VREFINEXT.=−125
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per PhaseMAX8686
REFIN
RREFIN
VEXT
